Many chemical reactions occurring during the process in which cells uptake nutrients and energy is taken out by metabolism greatly depend on temperature. In other words, it can be said that various functions and reactions of cells are controlled by intracellular or extracellular temperature. For example, researches using physical properties of temperature are frequently found in medical research, and abnormal thermogenesis in cancer cells, etc. has been reported (Non Patent Document 1: Scand. J. Haematol. 1986; 36: 353-357). It is possible to use thermogenesis to distinguish between cancer cells with high metabolic activity and normal cells without such activity. By using this, development of therapies and drug discovery targeting thermogenesis has been studied.
Not only in medical research, temperature control is also important in food industry such as fermentation processes in which microorganisms are used to control cellular metabolic activity. For example, in brewing of liquors, the existence of correlation between heat production from yeast and production of ethanol has been studied (Non Patent Document 2: Biotech. Bioeng. 1989; 34: 86-101).
On the other hand, despite the fact that temperature is an important biological indicator and that many of important intracellular reactions occur intracellularly, there are few attempts to measure intracellular temperature. One reason for this was that temperature measurement methods for micro-regions at the cellular level have not been established. However, recent researches have begun to study the use of a molecule material with variable physical properties depending on change in temperature as a temperature sensor (Non Patent Document 3: Photochem. Photobiol. 1995; 62: 416-425; Non Patent Document 4: J. Phys. D: Appl. Phys. 2004; 37: 2938-2943; Non Patent Document 5: Anal. Chem. 2006; 78: 5094-5101; Non Patent Document 6: Appl. Phys. Lett. 2005; 87: 201102; Non Patent Document 7: Biophys. J. 1998; 74: 82-89), and in particular, a high-performance molecular temperature sensor with a minimum functional unit of one molecule was developed and its application to cells has begun to be studied.
There has already been reported, as an example of a molecular temperature sensor, a fluorescent temperature sensor based on the principle that a 2,1,3-benzoxadiazolyl group as an environmentally responsive fluorophore is incorporated into polyacrylamide as a thermoresponsive polymer (Non Patent Document 8: Anal. Chem. 2003; 75: 5926-5935). This polymeric fluorescent temperature sensor has a property that the fluorescence intensity increases with a rise in temperature in an aqueous solution by a phase transition of polyacrylamide induced by heat, thus making it possible to observe a change of temperature in the system by measuring the fluorescence intensity.
There has also been reported the use of a microgel of polyacrylamide containing a fluorophore incorporated thereinto, which is obtained by adding a cross-linking agent during production of a polymer, as a fluorescent temperature sensor (Non Patent Document 9: J. Mater. Chem. 2005; 15: 2796-2800; Non Patent Document 10: J. Am. Chem. Soc. 2009; 131: 2766-2767). Furthermore, it has been reported that incorporation of an ionic functional group into a polymer expands the measuring temperature responsive range by inhibiting the aggregation of polymer (Patent Document 1: PCT/JP 2008/029770).
It has already been reported that the temperature range in which a fluorescent polymeric sensor responds in an aqueous solution can be selected by changing the structure of acrylamide as the main chain used as a thermoresponsive units (Non Patent Document 11: Anal. Chem. 2004; 76: 1793-1798), and it was also possible that a plurality of acrylamide compounds are used for a thermoresponsive unit for the temperature responsive range of interest.